Devices for applying pressure to an object to be pressurized via a pressure rod include a device that carries out pressurization by converting fluid energy such as compressed air into linear motion of the pressure rod. Generally, such a pressure device is called a fluid pressure cylinder. The basic structure thereof comprises a cylinder tube in which a piston is housed to be reciprocable axially, end covers provided at both ends of the cylinder tube, and a pressure rod protruding from an end of a cylinder body formed by the cylinder tube and the end covers. The pressure rod is attached to the piston, and when fluid pressure is supplied to a pressure chamber formed in the cylinder body, the piston and the pressure rod are driven axially.
Among fluid pressure cylinders, a fluid pressure cylinder of a type in which pressure chambers are formed on both sides of the piston and advance movement and retraction movement of the pressure rod are performed by fluid pressure is called a double acting type, and the type in which one of the advance movement and the retraction movement is carried out by the fluid pressure and the other is carried out by an external force such as a spring force is called a single acting type. There are the cases where compressed air is used as the fluid energy for driving the piston, and where liquid such as working oil is used. In order to ensure airtightness between an outer circumferential surface of the piston and an inner circumferential surface of the cylinder tube, a sealing member such as an O ring or a packing is attached to an outer circumference of the piston.
In such a fluid pressure cylinder, when the piston and the pressure rod are reciprocated in a vertical direction or an inclined direction to apply pressure to an object, weight of the piston and the pressure rod acts on the object. Moreover, along with the reciprocation of the piston and the pressure rod, sliding friction is caused between the sealing member attached for enhancing the airtightness and the inner circumferential surface of the cylinder tube. When the object to be pressurized is intended to be subjected to a predetermined pressure thrust, the gravity and the friction resistance force working on the piston and the pressure rod become disturbance with respect to a target value and make it difficult to control the pressure thrust. Particularly, in a pneumatic cylinder operated by lower pressure than that in a hydraulic cylinder, an influence of the friction resistance of the sealing member becomes relatively large and, as a result, it becomes difficult to control the pressure thrust with high accuracy.
In order to achieve high accuracy of the manufacture products at, for example, production sites of precision apparatuses, a dust-free room (clean room), in which particles and aerosols floating in air are controlled at predetermined values or less, is used sometimes. If a conventional fluid pressure cylinder is used in the dust-free room, the sliding friction is generated and further a portion of the sealing member, which is in slidable contact with the piston or pressure rod, is peeled and floats, thereby becoming cause of dust, so that it is difficult to make management of the dust-free room.
An object of the present invention is to provide a pressure device capable of controlling the pressure thrust with high accuracy.
Another object of the present invention is to provide a pressure device for preventing dust generated in the device from being dispersed to the outside.